1. Field of the Invention
The present invention relates generally to an optical window which transmits infrared radiation. More particularly, the present invention relates to an infrared-transmissive optical window in which the effect of microwave radiation is minimized.
2. Description of Related Art
In various optical systems, it is frequently desired that the optical window be able to selectively transmit or reflect particular types of radiation, such as infrared, visible, or microwave, depending on the particular purpose of the system. For example, if the window provides optical access to a sensor which detects infrared radiation, the window must have a high transmission for infrared radiation. At the same time, the window must be able to prevent the transmission of radiation which would have a negative effect on the sensor behind the window. Various types of optical filters are known in the art for accomplishing this purpose, and include absorption filters, reflective multiple layer dielectric filters, and diffraction filters generated by optical holographic techniques, each of which is discussed below.
The absorption filter comprises a material which is impregnated with absorption dyes or materials having intrinsic absorption at the wavelength of the radiation which it is desired to absorb. However, the dye itself also decreases the amount of the desired radiation which is transmitted.
The reflective multiple layer dielectric filters typically consist of alternate layers of two dielectric materials having different refractive indices, which are deposited on the surface of a substrate. When the optical thickness of each layer is chosen to be one-quarter of the wavelength of the radiation being reflected, the structure is referred to as a "quarterwave stack". However, the numerous abrupt interfaces between successive layers in such a stacked structure may have defects which cause unwanted optical scattering, excessive absorption of radiation which leads to thermal damage, localized electric fields, and a tendency to delaminate.
Diffraction optical elements may be formed by optical holography in photosensitive gelatin materials. However, these structures have environmental stability problems and are susceptible to degradation by humidity and heat. Moreover, gelatin filters are limited to use for radiation in the visible to the near infrared range since sensitized gelatin is not sensitive to longer wavelength exposure. Consequently, gelatin filters cannot be used for infrared applications.
One method for overcoming the previously noted difficulties in the prior art is described in U.S. Pat. No. 4,545,646 to Chern et al, assigned to the present assignee. Chern et al provided a graded index optical material comprising a single layer of a selected material in which the composition and therefore the refractive index of the material is varied in a periodic and continuous pattern as a function of the thickness of the layer. A preferred method for forming single-notch rugate filters of the type described by Chern et al is described in U.S. Pat. No. 4,915,476 to Hall et al, assigned to the present assignee. Hall et al provided an error-compensated method for insuring that the continuously varying refractive index profile of the material as deposited matches the desired or ideal profile. The structures of Chern et al and Hall et al are typically used to protect sensors from damage by laser radiation. While the structures of Chern et al and Hall et al work well for their intended purpose, they were not intended to meet the current need for optical filters which minimize the effects of microwave radiation.
Consequently, there is a current need in the field of optics for a structure which will selectively transmit radiation at desired wavelengths while minimizing the effects of microwave radiation.